TVRO receiver system with tracking filter for rejection of image frequencies

Abstract

A TVRO receiver for the reception of frequency modulated satellite broadcast signals is provided with the capability of efficiently rejecting image frequencies generated as the received signals undergo intermediate frequency translation. A TVRO tuner, which includes a super-heterodyne circuit having a voltage-controlled oscillator (VCO), means for supplying a controlling tuning voltage to the VCO, and a mixer for combining incoming 1st IF signals with the output frequency of the VCO to reduce the frequency of the 1st IF signals to a selected 2nd IF frequency, is provided with a tracking filter which is adapted to pass only the incoming 1st IF signals and effectively reject any image frequencies generated at the received signal frequencies. The tracking filter comprises at least one low-pass filter section and has a cut-off frequency which is controlled by the same tuning voltage used to regulate the output frequency of the VCO in such a way that the filter rejects frequencies exceeding the VCO output frequency. The filter is designed so that image frequencies, which are generated during the intermediate frequency translation stage as the sum of a given VCO output frequency and the selected 2nd IF frequency, fall substantially beyond the filter cut-off frequency and are thereby attenuated. The tracking filter allows adequate rejection of image frequencies without need for accurate tuning of the low-pass filter sections or precisely matched variable capacitance devices.

We claim: 
     
       1. A TVRO receiver for receiving frequency modulated video signals while rejecting image frequencies generated at received signal frequencies, the receiver comprising a tuner including a super heterodyne circuit having a voltage-controlled oscillator (VCO), means for supplying a first controlling variable tuning voltage to said VCO, and a mixer for combining incoming 1st IF signals with the output frequency of said VCO to reduce the frequency of the 1st IF signals to a selected 2nd IF frequency;   a tracking filter upstream of said mixer and having a cut-off frequency controllable by application of a controlling tuning voltage thereto for passing only said incoming 1st IF signals while rejecting any image frequency signals generated from said incoming signals; and   means for supplying a second controlling variable tuning voltage to said tracking filter for adjusting said cut-off frequency to have a value higher than the incoming 1st IF signals but less than that of the image frequencies.   
     
     
       2. The TVRO receiving of claim 1 wherein the 2nd IF frequency is sufficiently high to permit the output frequency of said VCO to be above the frequency range of the 1st IF signals. 
     
     
       3. The TVRO receiver of claim 2 wherein image frequencies are generated at frequencies equal to the sum of a current VCO output frequency and the selected 2nd IF frequency, and the tracking filter is a low-pass filter having a cut-off frequency which is higher than the incoming 1st IF signals frequencies but lower than the image frequencies corresponding to these 1st IF signal frequencies. 
     
     
       4. The TVRO receiver of claim 3 wherein said first controlling tuning voltage to said VCO is also supplied as the second tuning voltage to said tracking filter. 
     
     
       5. The TVRO receiver of claim 4 wherein the tracking filter comprises at least one T-section series L shunt-C filter, each T-section filter including first and second series inductances L 1  and L 2 , and a series connection of a capacitance C 1  and a varactor diode V 1  connected in parallel across the junction of inductances L 1  and L 2  and a ground plane, the anode end of V 1  being connected to capacitance C 1 , and the cathode end of V 1  being connected to the ground plane. 
     
     
       6. The TVRO receiver of claim 5 wherein the filter tuning voltage to the filter is applied to inductance L 1  through a bias feed resistance R 3  and a voltage divider network comprising a resistance R 4  connecting the tuning voltage to bias resistance R 3 , and a resistance R 5  connecting the junction of resistances R 4  and R 3  to the ground plane. 
     
     
       7. The TVRO receiver of claim 6 wherein the incoming 1st IF signals are fed to said first inductance L 1  of the tracking filter through a first DC blocking capacitance C 5 , the resulting filtered output is extracted from the second series inductance L 2  through a second blocking capacitance C 3 , and an RF bypass path is provided through a bypass capacitance C 4  between the junction of the bias resistance R 3  and divider resistance R 4 . 
     
     
       8. The TVRO receiver of claim 7 wherein the tracking filter comprises a cascade connection of two T-section low-pass filters. 
     
     
       9. The TVRO receiver of claim 1 wherein said 1st IF signals have a frequency of 950 to 1450 MHz and said second IF signals have a center frequency of at least 450 MHz. 
     
     
       10. In a TVRO receiver for receiving frequency modulated satellite signals while rejecting any image frequencies generated therefrom, the receiver comprising a tuner including a super heterodyne circuit having a voltage-controlled oscillator (VCO) with an output frequency controllable by a variable tuning voltage V TUNE  and means for applying the tuning voltage to said VCO, and a mixer for combining incoming 1st IF signals with the output of said VCO to reduce the frequency of the 1st IF signals to a selected 2nd IF frequency, the improvement comprising a tracking filter upstream of said mixer and adapted to pass only said incoming 1st IF signals and reject any image frequency signals generated from said incoming signals at frequencies exceeding said VCO output frequency, said filter comprising at least one low-pass filter section and having an adjustable cut-off frequency, said cut-off frequency being adjusted to correspond to said VCO output frequency by applying said variable VCO tuning voltage V TUNE  to said filter so that frequencies exceeding the VCO output frequency are rejected by said filter.   
     
     
       11. The improved TVRO receiver of claim 10 wherein the tracking filter comprises at least one T-section low-pass filter section in the form of a series-L shunt-C section, said filter section including a variable capacitance diode responsive to the tuning voltage supplied to the filter for varying the cut-off frequency of that filter. 
     
     
       12. The improved TVRO receiver of claim 11 wherein said 1st IF signals have a frequency of 950 to 1450 MHz and said 2nd IF signal frequency is sufficiently high to permit the output frequency of said VCO to be above the frequency range of the 1st IF signals. 
     
     
       13. The improved TVRO receiver of claim 11 wherein the 2nd IF signals have a center frequency of at least 480 MHz. 
     
     
       14. A TVRO receiver for receiving frequency modulated video signals while rejecting image frequencies generated at received signal frequencies, the receiver comprising a tuner including a super heterodyne circuit having a voltage-controlled oscillator (VCO), means for supplying a first controlling tuning voltage to said VCO, and a mixer for combining incoming 1st IF signals with the output frequency of said VCO to reduce the frequency of the 1st IF signals to a selected 2nd IF frequency;   a low-pass tracking filter having a controllable cut-off frequency for passing only said incoming 1st IF signals while rejecting any image frequencies generated therefrom, said image frequencies being generated at frequencies equal to the sum of a current VCO output frequency and the selected 2nd IF frequency, said tracking filter having a cut-off frequency which is higher than the incoming 1st IF signal frequencies but lower than the image frequencies corresponding to these 1st IF signal frequencies; and   means for supplying a second controlling tuning voltage to said tracking filter for adjusting said cut-off frequency to have a value less than that of said image frequencies.   
     
     
       15. The TVRO receiver of claim 14 wherein the controlling tuning voltage to said VCO is also supplied as the tuning voltage to said tracking filter. 
     
     
       16. The TVRO receiver of claim 14 wherein the tracking filter comprises a cascade connection of two T-section low-pass filters. 
     
     
       17. The TVRO receiver of claim 14 wherein said 1st IF signals have a frequency of 950 to 1450 MHz and said second IF signals have a center frequency of at least 450 MHz.